Small-sized motors are used widely in automotive electric equipment, such as automotive door locking-unlocking devices and motor-driven mirror retractors. Some of the small-sized motors are driven by means of turning on power for only a predetermined period of time. Such a small-sized motor stops rotating by the action of a mechanical stopper after achievement of a motor function, such as door locking, and before power is turned off. Usually, in order to reliably achieve a motor function under various conditions, a motor power ON period is set such that power is turned off after elapse of a slight period of time after the motor stops rotating (the motor is locked) by the action of a mechanical stopper. In other words, after a small-sized motor stops rotating by the action of a stopper, power is still supplied for a certain period of time. It has been revealed that, as a result of supply of power after the motor stops rotating, the motor vibrates and thus produces unusual noise. The produced unusual noise continues until motor power goes off.
FIG. 11 shows a brush structure disclosed in Patent Document 1. As illustrated, two magnets are fixed on the inner circumferential surface of a yoke. In order to yield a magnetic-noise reduction effect through reduction of magnetic excitation force, Patent Document 1 discloses a brush structure in which the center of a second brush of paired brushes in contact with a commutator is circumferentially offset by a predetermined angle θ from a circumferential position lying on a diameter passing through the center of a first brush. Patent Document 1 proposes measures against motor noise through improvement of the brush structure. However, Patent Document 1 does not disclose measures against unusual noise to be produced at the time of motor lock.
Patent Document 1: Japanese Patent Application Laid-Open (kokai) No. S62-230340